Hummingbirds are amazingly small and fast. The filmmakers had to use a high-def, high-speed camera to capture these kinds of shots. Two-hundred to five-hundred frame-per-second shots allow you to see what the birds are really up to.
Looking at their computers, the filmmakers were able to see hummingbirds hitting each other on the heads. When they shot it, it just looked like flurries of beaks and wings. But later they found a hidden life that they could not have imagined.